Relative Rate and Product Studies of the Reactions of Atomic Chlorine with Tetrafluoroethylene, 1,2-Dichloro-1,2-difluoroethylene, 1,1-Dichloro-2,2-difluoroethylene, and Hexafluoro-1,3-butadiene in the Presence of Oxygen

Rate coefficients <i>k</i>₁–<i>k</i>₃ have been measured for Cl atom reactions with CF₂CF₂, CFClCFCl, and CCl₂CF₂ relative to <i>k</i>₄ for CF₂CF–CFCF₂ at 293 ± 2 K. <i>k</i>₄ was remeasured relative to Cl + ethane. Cl was generated by UV photolysis of Cl₂, and other species were monitored by FT-IR spectroscopy. The measurements yield <i>k</i>₁ = (6.6 ± 1.0) × 10^–11, <i>k</i>₂ = (6.5 ± 1.0) × 10^–11, and <i>k</i>₃ = (7.1 ± 1.1) × 10^–11 cm³ molecule^–1 s^–1, respectively, and <i>k</i>₄ = (8.0 ± 1.2) × 10^–11 cm³ molecule^–1 s^–1 is proposed. These results are discussed in the context of atmospheric chemistry. Subsequent chemistry in the presence of oxygen leads to oxygenated products that are identified via their IR spectra, and possible mechanisms are discussed. The yield of CF₂O from C₂F₄ is 93 ± 7%. Dichlorofluoroacetyl fluoride (CCl₂FCFO) was observed as a product from CFClCFCl, and chlorodifluoroacetyl chloride (CClF₂CClO) was observed from CCl₂CF₂ oxidation. C₄F₆ led to 66 ± 5% CF₂O and 38 ± 3% OCF₂CFC­(F)O. Reaction enthalpies and enthalpy barriers computed via CBS-QB3 theory help rule out some unfavorable mechanistic steps

Investigating the Elusive Nature of Atomic O from CO₂ Dissociation on Pd(111): The Role of Surface Hydrogen

CO2 dissociation is a key step in CO2 conversion reactions to produce value-added chemicals typically through hydrogenation. In many cases, the atomic O produced from CO2 dissociation can potentially block adsorption sites or change the oxidation state of the catalyst. Here, we used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations to investigate the presence of surface species from the dissociation of CO2 on Pd(111). AP-XPS results show that CO2 was dissociated to produce adsorbed CO, but dissociated atomic O was not observed at room temperature. We were only able to observe atomic O when CO2 was introduced at 500 K. Further investigations of O-covered Pd(111) revealed that chemisorbed O could be easily removed by low pressures of CO and H2. Notably, the effect of H2 is quite prominent since it could react with chemisorbed O at a pressure as low as 2 × 10–9 Torr, and the presence of H2 at ambient pressure prevented CO2 dissociation. DFT calculations showed that in the presence of background H2, facile CO2 dissociation took place via the reverse water–gas shift (rWGS) reaction, which resulted in the formation of adsorbed CO and removal of O by H2. DFT also identified the possible variation of surface species on simultaneous exposure of CO2 and H2 over Pd(111) depending on temperature and pressure, which opens alternative opportunities to tune the CO2 hydrogenation catalysis by controlling the reaction conditions

Selective Methane Oxidation to Methanol on ZnO/Cu₂O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/Cu2O/Cu­(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambient-pressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu2O/Cu­(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO–Cu2O step sites are active centers for methanol synthesis when exposed to CH4 and O2 due to an effective O–O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH4 → CH3OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu2O/Cu­(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity

Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechanochemical Synthesis of Pd–CeO₂

The methane dry reforming (DRM) reaction mechanism was explored via mechanochemically prepared Pd/CeO2 catalysts (PdAcCeO2M), which yield unique Pd–Ce interfaces, where PdAcCeO2M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO2 (PdCeO2IW). In situ characterization and density functional theory calculations revealed that the enhanced chemistry of PdAcCeO2M can be attributed to the presence of a carbon-modified Pd0 and Ce4+/3+ surface arrangement, where distinct Pd–CO intermediate species and strong Pd–CeO2 interactions are activated and sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CHx to CO, identified on PdAcCeO2M using isotope labeled diffuse reflectance infrared Fourier transform spectroscopy and highlighting linear Pd–CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm–1] species as key DRM intermediates, stemming from associative CO2 reduction. The milled materials contrast strikingly with surface processes observed on IW samples (PdCeO2IW) where the competing reverse water gas shift reaction predominates

In Situ Elucidation of the Active State of Co–CeO_<i>x</i> Catalysts in the Dry Reforming of Methane: The Important Role of the Reducible Oxide Support and Interactions with Cobalt

The activation of methane and its dry reforming with CO₂ was systematically studied over a series (2–30 wt %) of Co (∼5 nm in size) loaded CeO₂ catalysts, with an effort to elucidate the interplay between Co and CeO₂ during the catalytic process using in situ methods. The results of in situ time-resolved X-ray diffraction (TR-XRD) show a strong interaction of methane with the CoO_<i>x</i>–CeO₂ systems at temperatures between 200 and 350 °C. The hydrogen produced by the dissociation of C–H bonds in methane leads to a full reduction of Co oxide, Co₃O₄ → CoO → Co, and a partial reduction of ceria with the formation of some Ce³⁺. Upon the addition of CO₂, a catalytic cycle for dry reforming of methane (DRM) was achieved on the CoO_<i>x</i>–CeO₂ powder catalysts at temperatures below 500 °C. A 10 wt % Co–CeO₂ catalyst was found to possess the best catalytic activity among various cobalt loading catalysts, and it exhibits a desirable stability for the DRM with a minimal effect of carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated under reaction conditions by in situ time-resolved XRD and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed dynamic evolutions in the chemical composition of the catalysts under reaction conditions. CO₂ attenuated the reducing effects of methane. Under optimum CO- and H₂-producing conditions, both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO, both supported on a partially reduced ceria (Ce³⁺/Ce⁴⁺). We identified the importance of dispersing Co, anchoring it onto the ceria surface sites, and then utilizing the redox properties of CeO₂ for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and likely the interfacial sitee are essential to successfully close the catalytic cycle